The present invention relates generally to methods for protecting metallic surfaces from corrosion and, more specifically, to a method for protecting the internal and/or external metallic surfaces, such as aluminum alloy propellant tanks or other components, from corrosion due to exposure to environmental moisture, salt and warm and extreme cold with a coating, which is referred to herein as an inorganic paint for metal corrosion protection (IMCP).
Various metallic surfaces exposed to environmental conditions are prone to corrosion. The internal and external surfaces of propellant tanks and other space hardware are generally constructed of lightweight aluminum alloys, which are especially prone to corrosion. Such alloys may include the 2014-T6 aluminum alloy which is susceptible to corrosion. Propellant tanks and associated hardware, which are typically launched from coastal locales or even from ships, may be exposed to humidity and extreme temperatures, as well as to salt fog and sunlight during transportation and while on the launch pad, awaiting use. Such conditions may accelerate corrosion of aluminum alloys and other metals used in the propellant tanks and associated hardware. Corrosion of internal and external surfaces may cause pitting, cracking and other degradation that substantially affect the integrity of the surfaces. Most conventional anti-corrosive paints, such as organic-based paints, cannot be used on propellant tank surfaces because such paints are incompatible with certain fuels, such as liquid oxygen, and could create a risk of fire or explosion. Chromate conversion spray coatings, while compatible with liquid oxygen, require treatment with large quantities of coating material over a longer period of time within an enclosed area. The present invention provides an improved method for protecting metal surfaces from corrosion, involving treatment of metal surfaces using inorganic materials like a paint application, in a chemically controlled space, to form thin anti-corrosion protective layers that are compatible with liquid oxygen.
An object of the present invention is to provide a method for protecting the aluminum alloy and other metal surfaces of space launch vehicles/spacecrafts and other metal structures from corrosion that may occur when such surfaces are exposed to environmental conditions such as moisture, salt, and/or warm/extreme cold.
A further object of the invention is to provide a method for protecting aluminum alloy and other metal surfaces from corrosion by treatment with a corrosion-inhibiting substance that is compatible with a space vehicle fuel, such as liquid oxygen or liquid hydrogen or other fuels.
The present invention relates to a method for inhibiting corrosion of metal-containing surfaces, such as aluminum-alloy surfaces, that may occur when such surfaces are exposed to adverse environmental conditions. More specifically, the invention is directed to a method for inhibiting corrosion on such surfaces which involves generally the application of an inorganic aqueous alkali silicate corrosion-inhibiting solution followed by application of an aqueous inorganic sealant solution.
In one aspect of the invention, the method may involve contacting a first amount of aqueous silicate solution with a section of a metal surface to form a protective film. In a preferred embodiment, the aqueous silicate solution comprises sodium silicate. The metal surface may be comprised of aluminum alloy, or another similar least-resistant-to-corrosion metal (e.g., alloy/carbon steel, magnesium alloy, etc.). The method further involves the step of permitting the aqueous silicate solution to dry. Following drying, the method involves the step of applying a first amount of sealant solution to the portion of the metal surface contacted with the silicate solution. A portion of the sealant solution adheres to the silicate protective film and fills micro porosities that may exist in the protective film, forming a sealant layer. Such micro porosities have previously allowed small areas of the metal surface to be exposed to the atmosphere, thus permitting corrosion, especially when the metal surface is exposed to environmental conditions for extended periods. The sealant layer of the present invention coats the metal at the bottom of the porosities and/or plugs or caps the porosities to protect the metal at the bottom from exposure to the atmosphere. After the step of applying, the method includes the step of permitting the sealant layer to dry. In a preferred embodiment, the sealant solution comprises aqueous sodium dichromate solution, and the sealant layer comprises a dichromate compound, and the resultant coating is referred to as IMPC.
In another aspect of the invention, the method also includes the step, before the step of contacting an amount of aqueous silicate solution with a section of the metal surface, of cleaning the section of the metal surface to be contacted with aqueous silicate solution to remove any impurity on the metal surface. In a preferred implementation of this step of the invention, the cleaning step produces or results in a water break free surface. The method may also include the step of evaluating the metal surface to determine whether it is a water break free surface. If the metal surface is not a water break free surface, then the method includes applying a cleaner with the objective of providing a water break free surface.
In a further aspect of the invention, the method also includes the step, after the step of applying a first amount of sealant solution, and before the sealant solution dries, of rinsing the metal surface with an amount of water, preferably deionized water, with the objective of removing a portion of sealant solution that may not have adhered to the protective silicate film, and sealed porosities within the protective film. In another implementation, the method may include a step of lightly wiping the metal surface using a water dampened clean cloth to remove sealant solution that may not have adhered to the protective silicate film and sealed porosities. Deionized water is preferred.
The present invention is adaptable to various metal surfaces that may be subject to corrosion, including internal and external metal surfaces of space launch vehicles and spacecraft, as well as tanks, pipes, bins, and silos or other hardware. The aqueous silicate solution may be formed from any of several alkali metal silicates, although in the present invention, sodium silicate is the preferred source of silicate. The aqueous silicate and sealant solutions may be applied to the metal surfaces in various ways, such as by spraying, brush- or roller-painting, or immersing. A preferred embodiment of the present invention involves spraying a metal surface with the relevant solution in successive perpendicular passes.
The silicate protective film and preferred dichromate sealant layer are compatible with liquid oxygen, and so may be used with metal surfaces that may come in contact with liquid oxygen. In addition, they preferably may be applied with a simple pressure sprayer, in relatively low concentrations, over a short period of time, in the chemically controlled space.